This application is related to the following commonly assigned patents and patent applications.
U.S. Pat. No. 4,297,637--Crooks et al (1981) PA0 U.S. Pat. No. 4,318,043--Crooks et al (1982) PA0 U.S. Pat. No. 4,471,305--Crooks et al (1984) . PA0 U.S. Pat. No. 4,599,565--Hoenninger et al (this and above previous cases form a continuous connected chain of continuation-in-part applications dating from the July 20, 1978 filing date of U.S. Pat. No. 4,297,637) PA0 U.S. application Ser. No. 515,116, filed July 19, 1983 for Crooks PA0 U.S. application Ser. No. 515,957, filed July 21, 1983 for Crooks et al PA0 U.S. application Ser. No. 515,857, filed July 21, 1983 for Cannon et al
The entire disclosure contained in each of the above-identified related patents and patent applications is hereby expressly incorporated by reference.
As explained in the above-referenced related patents/applications, radio frequency spin echo NMR responses can be elicited from selected internal regions of an object under test by transmitting an initial NMR r.f. perturbation pulse into the object followed by at least one subsequent 180.degree. NMR r.f. nutation pulse (sometimes called a "flipping" pulse). Selected (typically planar) regions of the object under test are selectively activated at the appropriate NMR Lamor radio frequency by employing magnetic gradients oriented along x, y or z axes (or any other desired axis) and superimposed upon a static magnetic field during the transmitted r.f. perturbation and flipping pulses. Additional magnetic gradients are typically employed to correct for undesirable de-phasing effects during measurement cycles and/or to purposefully phase encode some NMR responses.
A spin echo NMR response occurs in accordance with the "rule of equal times" after each 180.degree. flipping pulse. Synchronous r.f. modulation/demodulation techniques are preferably employed and a rapid succession of individual NMR measurement cycles are typically repeated at the same or different repetition rates (e.g., 0.5 second and 1.0 second per cycle) during an overall or complete NMR image data acquisition cycle (e.g., typically in excess of 1 minute) from which sufficient NMR spin echo response data is collected so as to permit the generation of an NMR image of the selected internal region of an object under test. The NMR image data may be stored, displayed or otherwise used as desired and as will be appreciated. Typically such processing involves the combination of synchronously detected spin echo signals from plural individual NMR measurement cycles and the use of Fourier transformation(s) or other image reconstruction processes using the combined spin echo data thus acquired. The above-referenced related patents/applications teach several techniques for deriving NMR images and/or image data from NMR r.f. spin echo signals and such details will therefore not be repeated in the present application.
The principles and techniques of NMR imaging are also discussed in detail at Chapters 2, 3 and 4 of "NMR Imaging In Medicine" edited by L. Kaufman, L. Crooks and A. R. Margulis (Igaku-Shoin, New York, New York). Two of the possible NMR imaging techniques discussed in Chapter 3 use projection reconstruction (PR) and two-dimensional Fourier transform (FT) techniques.
To effect projection reconstruction, a linear magnetic gradient is established (e.g. during NMR excitation/response times) along an axis that is sequentially rotated by an increment angle so as to collect NMR image data representing projection views from correspondingly sequentially incremented angles which may then be filtered (e.g. with a digital convolution algorithm) and back-projected or otherwise processed into the picture elements (pixels) of an image plane using techniques similar to those used in computed x-ray tomography. There are many such known projection reconstruction algorithms. Such prior art back-projection image reconstruction algorithms are generally depicted at FIGS. 2 and 3A. As there shown, it is customary to obtain approximately N equally spaced projection views over a total 180.degree. angular interval with the raw data being successively gathered over monotonically increasing angles relative to some initial view angle.
Fourier transforms (or other similar transformation algorithms) may also be used to construct visual images from NMR responses. For example, as depicted at FIGS. 5 and 6A, a magnetic gradient (i.e. G.sub.Y along a Y axis) may have a controlled duration and amplitude so as to effect a desired increment of phase encoding during the NMR measurement sequence. As depicted in FIG. 3, the measurement sequence may involve a 90.degree. NMR excitation pulse followed by a 180.degree. NMR excitation pulse so as to produce (in accordance with the rule of equal times), an NMR spin echo SE response signal. Although a Y axis phase encoding magnetic gradient is schematically depicted in FIG. 5, it should be appreciated that other axes could also be utilized and that the phase encoding gradient could be applied at different portions of a given measurement sequence. Such phase encoding gradient is changed during successive NMR data measurement sequences so as to obtain a family of NMR image data responses which, when properly Fourier transformed, give rise to pixel values for an NMR image for the planar or other volume of nuclei from which NMR responses have been elicited. As shown in FIG. 5, the first measurement sequence typically has heretofore utilized a maximum phase encoding in one direction followed by successively less phase encoding in the following measurement sequences until a zero phase encoding measurement sequence occurs. Thereafter, successively increasing degrees of phase encoding in the opposite sense are utilized for a still further plurality of measurement sequences so as to give rise to the complete image data acquisition cycle comprising a family of NMR responses which can be subjected to Fourier transformation in accordance with known procedures to produce an NMR image.
In both PR and FT imaging techniques, the prior data acquisition sequences have involved successive procedures which only change by a small incremental amount (related to the desired resulting image resolution). In fact, in some computed tomography techniques, some overlapped scanning may be performed with averaging techniques utilized in an attempt to ensure that only a relatively continuous set of raw data is input to the imaging algorithm. If the object under test moves between the start and finish times of a complete image data gathering procedure, the resulting image will include motion artifacts resulting from an attempt to reconstruct a somewhat inconsistent noncontinuous set of input data.
In conventional computed x-ray tomography, such motion artifact may be minimized by patient breath-holding during a fairly rapid scan (e.g. 1-20 seconds) during which all of the x-ray image data is taken. However, such rapid collection of all NMR imaging data, at least in the near future, cannot be obtained in such a short time interval that a patient might be expected to continuously hold his/her breath (e.g. on the order of one minute or more is currently required for a complete NMR imaging procedure). Nevertheless, a patient might still be expected to hold his/her breath (or to otherwise remain substantially motionless) for the first 20 seconds or so of a longer NMR imaging data gathering routine.